The present invention relates to a process for producing 2,6-naphthalenedicarboxylic acid continuously by oxidizing 2,6-diisopropylnaphthalene. More in detail, the present invention relates to a process for producing 2,6-naphthalenedicarboxylic acid characterized by the process comprising:
(1) step of dissolving 2,6-diisopropylnaphthalene and a catalyst comprising a water-soluble salt of cobalt, a water-soluble salt of manganese or a mixture thereof, a water-soluble salt of cerium and a bromine compound in an aliphatic monocarboxylic acid as the solvent,
(2) steps of supplying the obtained solution continuously into a reaction vessel and of oxidizing 2,6-diisopropylnaphthalene by molecular oxygen under an elevated temperature and a pressure,
(3) step of drawing the reaction mixture continuously out from the reaction vessel, then depositing and separating crude 2,6-naphthalenedicarboxylic acid crystals from the reaction mixture,
(4) step of bringing the separated crystals into contact with an aqueous solution of a mineral acid, thereby dissolving and removing metals of the catalyst from the crystals,
(5) step of purifying the crude 2,6-naphthalenedicaboxylic acid crystals,
(6) step of adding an alkali carbonate or an alkali bicarbonate to the filtrate of the step of dissolving and removing the metals, thereby depositing and separating the metals as carbonates and/or bicarbonates,
(7) step of supplying the separated carbonates and/or bicarbonates of the metals to the step of dissolving 2,6-diisopropylnaphthalene and the catalyst, and
(8) step of supplying the filtrate of the step of separating the crude 2,6-naphthalenedicarboxylic acid crystals into the step of dissolving 2,6-diisopropylnaphthalene and the catalyst.
Hitherto, as the process for producing 2,6-naphthalenedicarboxylic acid (hereinafterreferred to as 2,6-NDCA), a process for oxidizing 2,6-dimethylnaphthalene or 2,6-diisopropylnaphthalene by molecular oxygen in acetic acid as the solvent in the presence of a catalyst comprising cobalt and/or manganese and bromine has been known [refer to Japanese Patent Publications No. 48-43893 (1973), No. 56-21017 (1981), No. 59-13495 (1984) and No. 48-27318 (1973) and Japanese Patent Application Laid-Open (KKKAI) Nos. 48-34153 (1973), 49-42654 (1974), 52-17453 (1977), 60-89445 (1985) and 60-89446 (1985)].
Among the processes disclosed in the above references, particularly the following two processes are remarkable:
(1) Process disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 60-89445 (1985):
Process for producing 2,6-NDCA by oxidizing 2,6-diisopropylnaphthalene or its oxidized intermediate with molecular oxygen in a solvent containing at least 50% by weight of an aliphatic monocarboxylic acid of less than 3 carbon atoms, wherein the oxidation of 2,6-diisopropylnaphthalene or its oxidized intermediate is carried out in the presence of a catalyst comprising (i) cobalt and/or manganese and (ii) bromine in the ratio of at least 0.2 mol of the heavy metal to one mol of 2,6-diisopropylnaphthalene or its oxidized intermediate.
(2) Process disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 60-89446 (1985):
Process for producing 2,6-NDCA by oxidizing 2,6-diisopropylnaphthalene or its oxidized intermediate with molecular oxygen in a solvent containing at least 50% by weight of an aliphatic monocarboxylic acid of less than 3 carbon atoms, wherein the oxidation of 2,6-diisopropylnaphthalene or its oxidized intermediate is carried out in the presence of a catalyst comprising (i) cobalt and/or manganese and (ii) bromine and containing cobalt and/or manganese in an amount of at least 1% by weight of the aliphatic monocarboxylic acid of less than 3 carbon atoms.
However, in the above processes, not only a large amount of impurities, for instance, aldehyde derivatives, ketone derivatives, colored substances and nuclear bromides but also the derivatives of phthalic acid and trimellitic acid due to the cleavage of the naphthalene ring are formed, thereby not only the yield of 2,6-NDCA is reduced but also it becomes necessary to provide a complicated purifying process. Moreover, since 2,6-NDCA is obtained with by-products of the oxidation reaction, such as, aldehydes, ketones, brominated derivatives and oxidized polymers of 2,6-NDCA and colored substances, when such 2,6-NDCA is used as a starting material for producing polyethylene 2,6-naphthalate, polyester, polyamide, etc., the degree of polymerization of the polymers become low and the physical properties such as heat-resistance and the appearance such as color of the films and fibers prepared from the polymers are damaged.
Accordingly, as the purifying method of 2,6-NDCA, the following methods have been proposed:
(1) A method comprising the steps of dissolving crude 2,6-NDCA in an aqueous alkali solution, subjecting the solution to thermal treatment for from 1 to 5 hours at a temperature from 100.degree. to 250.degree. C. by stirring thereof, subjecting the solution to decoloring treatment by a solid adsorbent and blowing an acidic gas such as gaseous carbon dioxide or gaseous sulfur dioxide into the solution under a pressure, thereby reducing the pH of the solution and precipitating 2,6-NDCA as a monoalkali salt from the solution [refer to Japanese Patent Publication No. 52-20993 (1977)].
(2) A method comprising the steps of treating an aqueous alkali solution of crude 2,6-NDCA with an oxidizing agent such as alkali perhalogenate or alkali permanganate and blowing gaseous carbon dioxide or gaseous sulfur dioxide into the solution, thereby separating 2,6-NDCA as a monoalkali salt [refer to Japanese Patent Application Laid-Open (KOKAI) No. 48-68554 (1973)], and
(3) A method wherein after dissolving crude 2,6-NDCA into an aqueous solution of sodium acetate, condensing the solution and carrying out deposition of crystals, thereby isolating 2,6-NDCA as a monoalkali salt [refer to Japanese Patent Application Laid-Open (KOKAI) No. 50-105639 (1975)].
By the way, words "nuclear bromide(s)" in this application mean an aromatic compounds of which hydrogen(s) of aromatic nucleus is substituted by bromine(s), such as bromobenzene, bromonaphthalene, etc.
However, every one of the above methods of purification is using a method wherein crude 2,6-NDCA is dissolved in an aqueous alkali solution and crystals of the monoalkali salt of 2,6-NDCA are precipitated by adjusting pH of the solution. In the method of purifying crude 2,6-NDCA by adjusting pH of the solution, it is adjusted to from 6.5 to 7.5 by blowing gaseous carbon dioxide or sulfur dioxide under a pressure into an alkali solution of a relatively high concentration of 2,6-NDCA while warming it or by adding a mineral acid to the solution. Then the solution is cooled to 20.degree. C. and the monoalkali salt of 2,6-NDCA is precipitated. The method has a demerit that the composition and the amount of the crystals are variable, are not stable, depending on the conditions such as pH, temperature and concentration, because there exists a delicate equilibrium relationship between the monoalkali salt, dialkali salt and free acid of 2,6-NDCA.
Further, as other carboxylic acids having pKa close to that of 2,6-NDCA are contained in 2,6-NDCA obtained by oxidizing 2,6-dialkylnaphthalene, it is difficult to purify crude 2,6-NDCA to a high purity only by means of adjusting pH. Moreover, it is necessary to wash the crystals with water to remove the mother liquor accompanied, after separating the monoalkalisalt precipitated by pH adjustment. However, since the monoalkali salt of 2,6-NDCA is soluble in water, there is a defect that the rate of recovery of 2,6-NDCA is reduced by the washing.
As it is impossible to purify crude NDCA to a high purity only by crystallization, it is necessary to combine the method of crystallization with other methods such as thermal treatment, oxidative treatment or reductive treatment.
However, when a crystallization is combined with thermal treatment, which makes high temperature and pressure inevitable, oxidative reaction or reductive reaction, there is a problem of numbers of newly formed by-products which become impurities, resulting in a necessity of a means of removing the impurities. Accordingly, the combined method is incomplete as the method of purifying the crude 2,6-NDCA.
Still more, cobalt which is used as a component of the catalyst in the production of 2,6-NDCA is an expensive heavy metal which is relatively difficult to obtain. Accordingly, it is important industrially to reduce the amount of cobalt as small as possible, however, when the amount of cobalt is reduced in the conventional method, the amount of formation of trimetllitic acid, etc. are increased and the yield and purity of 2,6-NDCA are reduced, therefore, it has been recommended to use cobalt in a large amount in the reference.
Moreover, since crude 2,6-NDCA obtained by a conventional method accompanies nuclear bromides [refer, for instance, Japanese Patent Application Laid-Open (KOKAI) No. 48-96573 (1973) and Example 1 of Japanese Patent Application Laid-Open (KOKAI) No. 48-68555 (1973)], from 1000 to 2000 ppm of bromine is usually contained in the product. Also in the present inventors' experiments in the production of 2,6-NDCA by the conventional method, the similar results are obtained and there are many cases wherein from 2000 to 4000 ppm of bromine is contained in the product depending on the reaction conditions. It has been known that the softening point of polyethylene naphthalate produced by using 2,6-NDCA containing large amount of nuclear bromides has been reduced and as a result, quality of the polymer becomes poor.
Furthermore, the conventional process for producing 2,6-NDCA is performed batch-wise, and although possibility of applying a continuous process for producing 2,6-NDCA is suggested, no concrete proposal has been given so far.
In the conventional processes for producing 2,6-NDCA, a large quantity of by-products and decomposed products such as trimellitic acid, aldehydes, colored substances and nuclear bromides is formed, and accordingly a large quantity of heavy metal salt of trimellitic acid and nuclear bromide is contained in the crude 2,6-NDCA and the purity of the crude 2,6-NDCA is usually about 80%. Accordingly, in order to obtain 2,6-NDCA of a purity of higher than 99% from such crude 2,6-NDCA, a complicated purifying method with many steps is necessary, and in addition, since an expensive cobalt has been used in a large amount, the conventional process is unsatisfactory as an industrial process for producing 2,6-NDCA. NDCA.
Moreover, in the method for purifying the crude 2,6-NDCA, a chemical reactions and operations such as condensation, cooling, etc. are performed, therefore, it is impossible to obtain 2,6-NDCA of a high purity in a high rate of recovery.
Still more, in the production of 2,6-NDCA in an industrial scale, since the conventional process is carried out batch-wise, it is impossible to produce a large quanttty of highly pure 2,6-NDCA. Accordingly, an proposal of a continuous process for producing 2,6-NDCA of a purity of higher than 98% in a large quantity has been demanded.
As a result of the present inventors' studies on the process for producing 2,6-NDCA, they have found the way to control the amount of formation of trimellitic acid and nuclear bromides, among many by-products, which give an important influence on the recovery of the heavy metals (cobalt and/or manganese), the re-use thereof and the yield and the purifying steps of 2,6-NDCA while using a far smaller amount of cobalt catalyst than that has been used, and accordingly, the formation of trimellitic acid is suppressed and 2,6-NDCA can be produced in a favorable yield by adding cerium to the catalyst comprising cobalt and/or manganese and bromine. In addition, they have developed the continuous process for producing highly pure 2,6-NDCA in a large quantity and also at a moderate cost, the process is comprising of the following steps:
(1) the step for dissolving 2,6-diisopropylnaphthalene and a catalyst comprising a water-soluble salt of cobalt, a water-soluble salt of manganese or a mixture thereof, a water-soluble salt of cerium and a bromine compound in an aliphatic monocarboxylic acid as the solvent,
(2) the step for oxidizing 2,6-diisopropylnaphthalene by molecular oxygen under an elevated temperature and a pressure, while continuously supplying the solution into a reaction vessel,
(3) the step for drawing the reaction mixture continuously out from the reaction vessel and precipitating and separating crude 2,6-NDCA from the reaction mixture,
(4) the step for bringing the crude 2,6-NDCA crystals separated into contact with an aqueous solution of a mineral acid, thereby dissolving and removing the metals of the catalyst from the crystals,
(5) the step for purifying the crude 2,6-NDCA crystals,
(6) the step for adding an alkali carbonate or an alkali bicarbonate to the filtrate from the step of dissolving and removing the metals, thereby precipitating and separating the metals as carbonates and/or bicarbonates,
(7) the step for supplying the carbonate and/or bicarbonate of the metals to the step for dissolving 2,6-diisopropylnaphthalene and the catalyst, and
(8) the step for supplying the filtrate from the step of separating the crude 2,6-NDCA crystals to the step for dissolving 2,6-diisopropylnaphthalene and the catalyst.
On the basis of the above findings, the present inventors have completed the present invention.